Since the chemical structures of synthetic fibers used in the cord fabric production and the rubber are considerably different from each other, the said materials are incompatible with each other in terms of their chemical and physical structures. The synthetic fibers have high strength and low elongation whereas the rubbers contrarily are polymeric materials which have high elongation and low strength. The polar groups (amide, hydroxyl and carbonyl groups) present in the structure of the synthetic fibers are incompatible with the non-polar structures of the rubber. This incompatibility is eliminated with water-based Resorcinol-Formaldehyde-Latex (RFL) adhesive solutions which form a phase between the cord and the rubber and enable the rubber and the fiber to be attached to each other.
Main function of RFL is to serve as adhesive attaching two incompatible structures by forming phase between fiber and rubber. RF functional group within RFL is attached to the polar groups of fiber, Latex (L) group is attached to the fiber with vulcanization and the rubber-fiber composite structure is formed. The vehicle tire application amongst the most significant product examples wherein the said composite structure is used. The water based RFL adhesives are applied on cord fabric during “dipping” process which is the final step of cord fabric production. The strength of the bonds formed between the rubber and the cord is examined with adhesion tests. Adhesion is very important parameter in high tenacity cord reinforced rubber products. Because cord-rubber adhesion is a critical factor which affects the tire performance.
Formaldehyde-free dipping solutions for rubber materials reinforced with cord have been reported in various studies.
U.S. Patent document no US20120041113, an application known in the state of the art, discloses preparing a composition comprising epoxy, blocked isocyanate, epoxy curing agent and vinyl pyridine latex.
International Patent document no WO9600749, another application known in the state of the art, discloses applying dipping solutions formulated with three functional epoxy resins, styrene-butadiene-vinyl pyridine and styrene-butadiene-acrylonitrile-acrylic acid latex to the polyethylene terephthalate (PET) material and adhesion strength with rubber.
U.S. Pat. No. 5,118,545, another application known in the state of the art, discloses the synthesis of an aramide with multiple double bonds. It is stated that the synthesized resin is applied on the aramide based cord fabric and the double bonds in aramide resin are vulcanized with the double bonds in rubber, and the amide groups provide physical adhesion to the aramide fiber.
U.S. Pat. No. 4,472,463, another application known in the state of the art, discloses dipping non-adhesive activated PET fibers with two step process dipping. The first step dipping comprises aromatic glycidyl ester epoxy and blocked isocyanate, while the second step dipping comprises two various latex. The first latex is styrene-butadiene-vinyl pyridine copolymer, and the other one is acrylic acid-methyl methacrylate-styrene copolymer. It is stated that higher adhesion values are obtained in H-adhesion tests compared to RFL dipping solution.
U.S. Patent document no US20040249053, another application in the state of the art wherein an environmental friendly dipping material is disclosed. In that application, the maleinized polybutadiene is reacted with polyethylene glycol and become water soluble. The PET cords modified with epoxy are first dipped with this resin and then with styrene-butadiene-vinyl pyridine latex. However the stripping-adhesion strength of the rubber with PET cords dipped with this resin is lower than the fabrics with RFL.
RFL adhesive formulation has been used as adhesive material in all synthetic fiber reinforced materials for over half century because of its stable structural characteristics and low cost. However, both resorcinol and formaldehyde are the hazardous chemicals which create a great risk for human and environmental health. Therefore, their use has to be limited. Regarding this subject, feedbacks have come from international organizations and customers. It is known that the resorcinol causes itching and rash when it contacts the skin, irritates the eye and shows toxic properties in liver and cardiovascular systems.
Formaldehyde is riskier than resorcinol for human health and safety. In 2004, formaldehyde is classified as group 2A chemical (possible carcinogen in humans) by a group of scientists in International Agency for research on Cancer (IARC) of World Health Organization, and as group 1 (carcinogen for humans) for a group of scientists. In 2009, formaldehyde is included in list of chemicals causing leukemia by IARC. According to these, formaldehyde is suggested to be gene mutagen. Even low amounts of formaldehyde (1 ppm) cause eye, nose and throat irritation.
Although formaldehyde resins have advantage in terms of cost, both producers and consumers search for alternatives because of the reasons expressed above.